The exemplary embodiments of this invention relate generally to Hall-effect sensors and, more specifically, to graphene-based Hall-effect sensors and the use thereof for the characterization and analysis of fluid flow at micro-scale or nanoscale levels.
Velocimetry is the measurement of fluid velocity. Velocimetric techniques at microscale levels, such as Microscopic Particle Image Velocimetry (microPIV) and Microscopic Particle Tracking Velocimetry (microPTV), use optical (e.g., laser) detection and are, therefore, limited to microchannels etched in transparent materials, such as glass, polydimethylsiloxane (PDMS), and polymethyl methacrylate (PMMA). Both microPIV and microPTV techniques rely on the excitation of fluorescent microbeads using intense laser light pulses. Both techniques also have several limitations that prevent their application to fluid flow on a nanoscale level.
Magnetic nanoparticles can be used in various velocimetric techniques as markers for biological assays or tracers for fluid flow characterization. However, nanoparticles are generally on the order of about 1 nanometer (nm) to hundreds of nanometers in diameter. As such, their reduced size, weak magnetic field, large surface-to-volume ratio, and thermal disturbance (superparamagnetism) introduce challenges for achieving detectability in moving fluids.
The fabrication and integration of sensors and generators of magnetic fields into micro-/nanosystems designed to work with nanoparticles may be a complex and difficult task. Use of sensors and generators may also require advanced detection strategies to compensate for poor signal-to-noise ratio in the detection methods. Such detection strategies can be implemented within a CMOS-compatible process, but they can also be implemented in arbitrary substrate devices.
Previous attempts to detect small magnetic particles with Hall sensors employed Si-, InSb-, and graphene-based Hall devices and were able to detect the presence of magnetic microbeads composed of thousands of nanometer-sized iron oxide particles dispersed in a polymer matrix. However, the detection of moving nanoparticles was generally not possible, since the microbeads had to be positioned with highly complex apparatuses (atomic force microscopes (AFM), nanomanipulators, etc.) precisely on top of the sensors, since there was no integration of the sensors into micro-/nanofluidic channels. Furthermore, the sensor areas, which are typically about 1 square micrometer (μm2) to about 6 μm2, made it difficult or even impossible to integrate such sensors into certain micro-/nanofluidic devices. Aside from the difficulties associated with integrating the sensors into micro-/nanofluidic devices, the detection of single nanoparticles that are a few nanometers in diameter has not been demonstrated. Moreover, methods to characterize flow properties of a fluid through micro-/nanofluidic channels based on the exploration of dispersed magnetic nanoparticles have not been successfully carried out.
Alternatively, an Al2O3-based Magnetic Tunnel Junction (MTJ) has been used as a sensor for the detection of magnetic microbeads. The integration of this sensor into a microfluidic channel allowed for the detection of moving microbeads as they rolled on top of the MTJ. It was not, however, demonstrated how this approach could be used to detect the presence of single nanometer-sized magnetic particles, nor how the dispersed particles could be used to better characterize the flow of the surrounding fluid. Additionally, the relatively large size (about 10 μm2) of the sensor hinders its integration into certain micro-/nanofluidic devices.
Along the same lines, MgO-based MTJs have been able to detect 2.5 micro molar (μM) target DNA labeled with 16 nm iron oxide nanoparticles. In doing so, the DNA strands were able to bond on the sensor surface. Provided the coverage was above a certain threshold, a signal was then detected denoting the presence of magnetic labels (nanoparticles). With this approach, it was not possible to detect single nanoparticles nor use them to characterize the flow of carrying fluid. It was also not possible to detect moving magnetic particles, since they had to be attached to the sensor surface for detection. Finally, the setup required a large array (4×104 μm2) of elliptical MTJ sensors, each one having a surface area of 85 μm2, which made it unsuitable for integration into certain micro-/nanofluidic devices.
Micrometer- and submicrometer-sized Hall sensors using graphene, InSb, and InAs/AlSb (2-dimensional electron gas) have also been characterized and optimized with regard to the detection of very small magnetic fields. The ability to detect a few magnetic nanoparticles with such devices has not been demonstrated. In fact, these attempts did not even anticipate the detection of a single nanoparticle. Furthermore, assumptions made to carry out such attempts were based on a perfect placement of nanoparticles on top of the sensor and did not provide a method to detect moving nanoparticles, nor to determine the flow properties of the carrying fluid.
Another attempt at single-nanoparticle detection employed giant magnetoresistance (GMR) spin valve sensors to detect a few tens to hundreds of 16 nm iron oxide nanoparticles. However, in such attempts it was not possible to detect a single nanoparticle of comparable size. The detection method required the nanoparticles to be bound to the sensor and, therefore, did not provide a way to characterize the flow of the surrounding fluid.
Submicrometric semiconductor-based Hall sensors have also been shown to perform single-nanoparticle detection in the case of 50-175 nm nanoparticles made of thousands of smaller (4 nm) FePt nanoparticles. The compound nanoparticle was positioned by an intricate operation that required the presence of a Si membrane, which ultimately limited the sensitivity of the device. The method was not suitable for detecting moving nanoparticles nor for characterizing the surrounding fluid via the magnetic tracers. The detection of a single nanoparticle of only a few nanometers (<50 nm) with such devices was not demonstrated.